In an electrophotographic printing apparatus, a conventional charging step can include applying a high voltage to a metal wire to generate a corona, which is then used for charging an electrophotographic photosensitive member. However, undesirable corona discharge products, such as ozone and NOx, are generated along with the corona. Such corona discharge products can adversely affect the photosensitive member surface, causing deterioration in, image quality such as image blurring or fading or they presence of black streaks across copy sheets, and can be harmful to humans if released in relatively large quantities—for example, ozone.
An alternative to corona charging is direct charging, where a contact type charging device is used. The contact type charging device can include an electrostatic charging member, such as a bias charging member, which is supplied with voltage and charges the photosensitive member when in contact with the photosensitive member. As such, bias charging members require an outer layer having surface resistivity within a desired range. Materials with insufficient (too low) resistivity will cause shorting and/or unacceptably high current flow to the photosensitive member. Materials with excessive (too high) resistivity will require disproportionately high voltages for charging. Other problems can also result if the resistivity falls outside of a desired range, including nonconformance at the contact nip, poor toner releasing properties, and generation of contaminants during charging due to leaching from the photosensitive member. These adverse effects can result in the electrostatic charging member having non-uniform resistivity across the length of the contact member or resistivity that is susceptible to variations in temperature, relative humidity, running time, and/or contaminants.
Due to direct contact with the photosensitive member, a contact type charging device is also subject to increased stress and mechanical degradation, typically resulting in surface defects—for example scratches, streaks, abrasions, and pothole-like deformations—on the electrostatic charging member. Defects on the electrostatic charging member can translate to unfavorable print defects in the final product, such as dark streaking and white/dark spots. These failures reduce the useful lifetime of an electrostatic charging member, and ultimately reduce the useful lifetime of an electrophotographic printing apparatus.
Thus, there is a need to overcome these and other problems, of the prior art and to provide electrophotographic charging members with desirable surface resistivity and improved mechanical strength, and to extend charging member lifetime utility.